Golf ball

ABSTRACT

A golf ball  2  has a plurality of dimples  12  and a land  14  on a surface thereof. The golf ball  2  further has a large number of minute projections  18  formed on surfaces of the dimples  12  and the land  14 . An average depth Fav of the dimples  12  and an average height Hav of the minute projections  18  satisfy the following mathematical formula (1). 
         Hav&lt;Fav *0.05  (1)
 
     An average value Pav of pitches P between the minute projections  18  and other minute projections  18  adjacent to the minute projections  18  is not less than 10 μm and not greater than 2000 μm.

This application claims priority on Patent Application No. 2016-166387 filed in JAPAN on Aug. 29, 2016. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls having dimples on the surfaces thereof.

Description of the Related Art

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.

JP2015-142599 (US2015/0182805) discloses a golf ball having a surface with large roughness. The roughness can be formed by blasting or the like. The roughness enhances the aerodynamic characteristic of the golf ball due to a synergetic effect with dimples.

JP2011-72776 (US2011/0077106) discloses a golf ball having a coating formed from a paint that contains particles. The particles enhance the aerodynamic characteristic of the golf ball due to a synergetic effect with dimples.

JPH2-68077 discloses a golf ball having dimples each having one projection at a bottom thereof. The dimples each having the projection enhance the aerodynamic characteristic of the golf ball.

The greatest interest to golf players concerning golf balls is flight distance. Golf players desire golf balls having excellent flight performance. Golf players having an average skill desire golf balls having excellent flight performance particularly when being hit with a long iron.

An object of the present invention is to provide a golf ball having excellent flight performance when being hit with a long iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention has a plurality of dimples and a land. The golf ball further has a large number of minute projections formed on surfaces of the dimples and/or the land. An average depth Fav of the dimples and an average height Hav of the minute projections satisfy the following mathematical formula (1).

Hav<Fav*0.05  (1)

With the golf ball according to the present invention, the minute projections suppress rising of the golf ball during flight. With the golf ball, a large flight distance can be achieved due to a synergetic effect of the dimples and the minute projections.

Preferably, an average value Pav of pitches P between the minute projections and other minute projections adjacent to the minute projections is not less than 10 μm and not greater than 2000 μm.

The golf ball can have a plurality of rows. Preferably, in each of the rows, a plurality of minute projections are aligned at equal pitches.

Preferably, an average area Sav of the dimples and an average area Qav of bottom surfaces of the minute projections satisfy the following mathematical formula (2).

Qav<Sav*0.016  (2)

The golf ball can include a main body and a paint layer positioned outside the main body. Preferably, the minute projections each have a shape in which a surface shape of the main body is reflected.

Preferably, an average value Lav of distances between the minute projections and other minute projections adjacent to the minute projections is not less than 5 μm and not greater than 1500 μm.

Preferably, the average height Hav of the minute projections is not less than 0.5 μm and not greater than 25 μm.

Preferably, a total number of the minute projections is not less than 500 and not greater than 500000.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;

FIG. 3 is a partially enlarged perspective view of the surface of the golf ball in FIG. 1;

FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1; and

FIG. 5 is a cross-sectional view taken along the line V-V in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The core 4, the mid layer 6, and the cover 8 are included in a main body 10 of the golf ball 2. The golf ball 2 has a large number of dimples 12 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 12 is a land 14. Although not shown in FIG. 1, the golf ball 2 further includes a later-described paint layer. The paint layer is positioned outside the main body 10. The main body 10 may have a one-piece structure. The main body 10 may have a two-piece structure, a four-piece structure, a five-piece structure, or the like.

The golf ball 2 preferably has a diameter of not less than 40 mm and not greater than 45 mm. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm. The diameter of the golf ball 2 according to the present embodiment is 42.7 mm.

The golf ball 2 preferably has a weight of not less than 40 g and not greater than 50 g. In light of attainment of great inertia, the weight is more preferably not less than 44 g and particularly preferably not less than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably not greater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of the base rubber of the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. Two or more rubbers may be used in combination. In light of resilience performance, polybutadienes are preferable, and high-cis polybutadienes are particularly preferable.

The rubber composition of the core 4 includes a co-crosslinking agent. Examples of preferable co-crosslinking agents in light of resilience performance include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. The rubber composition preferably includes an organic peroxide together with a co-crosslinking agent. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide.

The rubber composition of the core 4 may include additives such as a filler, sulfur, a vulcanization accelerator, a sulfur compound, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant. The rubber composition may include a carboxylic acid or a carboxylate. The rubber composition may include synthetic resin powder or crosslinked rubber powder.

The core 4 has a diameter of preferably not less than 30.0 mm and particularly preferably not less than 38.0 mm. The diameter of the core 4 is preferably not greater than 42.0 mm and particularly preferably not greater than 41.5 mm. The core 4 may have two or more layers. The core 4 may have a rib on the surface thereof. The core 4 may be hollow.

The mid layer 6 is formed from a resin composition. preferable base polymer of the resin composition is an ionomer resin. Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion.

Instead of an ionomer resin, the resin composition of the mid layer 6 may include another polymer. Examples of the other polymer include polystyrenes, polyamides, polyesters, polyolefins, and polyurethanes. The resin composition may include two or more polymers.

The resin composition of the mid layer 6 may include a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like. For the purpose of adjusting specific gravity, the resin composition may include powder of a metal with a high specific gravity such as tungsten, molybdenum, and the like.

The mid layer 6 has a thickness of preferably not less than 0.2 mm and particularly preferably not less than 0.3 mm. The thickness of the mid layer 6 is preferably not greater than 2.5 mm and particularly preferably not greater than 2.2 mm. The mid layer 6 has a specific gravity of preferably not less than 0.90 and particularly preferably not less than 0.95. The specific gravity of the mid layer 6 is preferably not greater than 1.10 and particularly preferably not greater than 1.05. The mid layer 6 may have two or more layers.

The cover 8 is formed from a resin composition. A preferable base polymer of the resin composition is a polyurethane. The resin composition may include a thermoplastic polyurethane or may include a thermosetting polyurethane. In light of productivity, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment.

The polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate.

The polyol, which is a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of an isocyanate for the polyurethane component include alicyclic diisocyanates, aromatic diisocyanates, and aliphatic diisocyanates. Alicyclic diisocyanates are particularly preferable. Since an alicyclic diisocyanate does not have any double bond in the main chain, the alicyclic diisocyanate suppresses yellowing of the cover 8. Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). In light of versatility and processability, H₁₂MDI is preferable.

Instead of a polyurethane, the resin composition of the cover 8 may include another polymer. Examples of the other polymer include ionomer resins, polystyrenes, polyamides, polyesters, and polyolefins. The resin composition may include two or more polymers.

The resin composition of the cover 8 may include a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like.

The cover 8 has a thickness of preferably not less than 0.2 mm and particularly preferably not less than 0.3 mm. The thickness of the cover 8 is preferably not greater than 2.5 mm and particularly preferably not greater than 2.2 mm. The cover 8 has a specific gravity of preferably not less than 0.90 and particularly preferably not less than 0.95. The specific gravity of the cover 8 is preferably not greater than 1.10 and particularly preferably not greater than 1.05. The cover 8 may have two or more layers.

The golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8. The reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8. The reinforcing layer suppresses separation of the cover 8 from the mid layer 6. The reinforcing layer is formed from a polymer composition. Examples of the base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.

The golf ball 2 has type I dimple specifications shown in Table 1 below. The contour of each dimple 12 is circular. The golf ball 2 has: dimples A each having a diameter of 4.60 mm; dimples B each having a diameter of 4.50 mm; dimples C each having a diameter of 4.30 mm; dimples D each having a diameter of 4.20 mm; dimples E each having a diameter of 4.00 mm; and dimples F each having a diameter of 2.90 mm. The number of kinds of the dimples 12 is six. The golf ball 2 may have non-circular dimples instead of the circular dimples 12 or together with circular dimples 12.

The number of the dimples A is 72; the number of the dimples B is 54; the number of the dimples C is 30; the number of dimples D is 54; the number of the dimples E is 108; and the number of the dimples F is 12. The total number of the dimples 12 is 330. A dimple pattern is formed by these dimples 12 and the land 14.

FIG. 2 shows a cross section of the golf ball 2 along a plane passing through the central point of the dimple 12 and the central point of the golf ball 2. In FIG. 2, the top-to-bottom direction is the depth direction of the dimple 12. In FIG. 2, a chain double-dashed line 16 indicates a phantom sphere. The surface of the phantom sphere 16 is the surface of the golf ball 2 when it is postulated that no dimple 12 and no minute projection (described in detail later) exist. The diameter of the phantom sphere 16 is equal to the diameter of the golf ball 2. The dimple 12 is recessed from the surface of the phantom sphere 16. The land 14 coincides with the surface of the phantom sphere 16.

In FIG. 2, an arrow Dm indicates the diameter of the dimple 12. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the far opposite ends of the dimple 12. Each tangent point Ed is also the edge of the dimple 12. The edge Ed defines the contour of the dimple 12.

The diameter Dm of each dimple 12 is preferably not less than 2.0 mm and not greater than 6.0 mm. The dimple 12 having a diameter Dm of not less than 2.0 mm contributes to turbulization. In this respect, the diameter Dm is more preferably not less than 2.5 mm and particularly preferably not less than 2.8 mm. The dimple 12 having a diameter Dm of not greater than 6.0 mm does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the diameter Dm is more preferably not greater than 5.5 mm and particularly preferably not greater than 5.0 mm.

In the case of a non-circular dimple, a circular dimple 12 having the same area as that of the non-circular dimple is assumed. The diameter of the assumed dimple 12 can be regarded as the diameter of the non-circular dimple.

In FIG. 2, a double ended arrow F indicates the depth of the dimple 12. The depth F is the distance between the deepest part of the dimple 12 and the surface of the phantom sphere 16. An average depth Fav is calculated by summing the depths F of all the dimples 12 and dividing the sum of the depths F by the total number of the dimples 12. In light of suppression of rising of the golf ball 2 during flight, the average depth Fav is preferably not less than 0.10 mm, more preferably not less than 0.13 mm, and particularly preferably not less than 0.15 mm. In light of suppression of dropping of the golf ball 2 during flight, the average depth Fav is preferably not greater than 0.50 mm, more preferably not greater than 0.45 mm, and particularly preferably not greater than 0.40 mm.

The area S of the dimple 12 is the area of a region surrounded by the contour line of the dimple 12 when the central point of the golf ball 2 is viewed at infinity. In the case of a circular dimple 12, the area S is calculated by the following mathematical formula.

S=(Dm/2)²*π

An average area Sav is calculated by summing the areas S of all the dimples 12 and dividing the sum of the areas S by the number of the dimples 12.

In the golf ball 2 according to the present embodiment, the area of each dimple A is 16.62 mm²; the area of each dimple B is 15.90 mm²; the area of each dimples C is 14.52 mm²; the area of each dimple D is 13.85 mm²; the area of each dimple E is 12.57 mm²; and the area of each dimple F is 6.61 mm². The average area Sav of the golf ball 2 is 14.17 mm².

In the present invention, the ratio of the sum of the areas S of all the dimples 12 relative to the surface area of the phantom sphere 16 is referred to as an occupation ratio So. From the standpoint that sufficient turbulization is achieved, the occupation ratio So is preferably not less than 78.0%, more preferably not less than 80.0%, and particularly preferably not less than 81.0%. The occupation ratio So is preferably not greater than 95%. In the golf ball 2 according to the present embodiment, the total area of the dimples 12 is 4675.6 mm². The surface area of the phantom sphere 16 of the golf ball 2 is 5728.0 mm², so that the occupation ratio So is 81.6%.

From the standpoint that a sufficient occupation ratio is achieved, the total number N of the dimples 12 is preferably not less than 250, more preferably not less than 280, and particularly preferably not less than 300. From the standpoint that each dimple 12 can contribute to turbulization, the total number N of the dimples 10 is preferably not greater than 450, more preferably not greater than 400, and particularly preferably not greater than 380.

In the present invention, the “volume of the dimple” means the volume of a portion surrounded by the surface of the phantom sphere 16 and the surface of the dimple 12. In light of suppression of rising of the golf ball 2 during flight, the total volume of all the dimples 12 is preferably not less than 450 mm³, more preferably not less than 480 mm³, and particularly preferably not less than 500 mm³. In light of suppression of dropping of the golf ball 2 during flight, the total volume is preferably not greater than 750 mm³, more preferably not greater than 730 mm³, and particularly preferably not greater than 710 mm³.

FIG. 3 is a partially enlarged perspective view of the surface of the golf ball 2 in FIG. 1. As is obvious from FIG. 3, the golf ball 2 has a large number of minute projections 18 on the surface thereof. As is obvious from FIG. 2, the minute projections 18 are formed on the surfaces of the dimples 12 and also on the surface of the land 14. Each minute projection 18 stands outward in the radial direction of the golf ball 2. The minute projections 18 suppress rising of the golf ball 2 during flight. The golf ball 2 having the minute projections 18 has excellent flight performance when being hit with a long iron. The minute projections 18 may be formed only on the surfaces of the dimples 12. The minute projections 18 may be formed only on the surface of the land 14.

FIG. 3 shows three minute projections 18 a belonging to a first row I, and three minute projections 18 b belonging to a second row II. The direction indicated by an arrow A in FIG. 3 is the direction in which the rows extend. In each row, the minute projections 18 are aligned at equal pitches. In other words, the minute projections 18 are regularly aligned. At a part of the surface of the golf ball 2, the minute projections 18 may be irregularly aligned.

The minute projections 18 a, which belong to the first row I, and the minute projections 18 b, which belong to the second row II, may be arranged zigzag. In other words, the positions of the minute projections 18 a, which belong to the first row I, may be displaced relative to the positions of the minute projections 18 b, which belong to the second row II, in the extending direction A.

FIG. 4 is a partially enlarged cross-sectional view of the golf ball 2 in FIG. 1. FIG. 4 shows the cover 8, which is a part of the main body 10, and a paint layer 20. FIG. 4 shows the minute projection 18. The cover 8 has a projection portion 22. The minute projection 18 is formed by the projection portion 22 and the paint layer 20. The projection portion 22 stands outward in the radial direction of the golf ball 2 (upward in FIG. 4). Thus, the minute projection 18 also stands outward in the radial direction of the golf ball 2. In other words, the minute projection 18 has a shape in which the surface shape of the main body 10 (cover 8) is reflected. In FIG. 4, reference sign 24 indicates the bottom surface of the minute projection 18.

FIG. 5 is a cross-sectional view taken along the line V-V in FIG. 4. FIG. 5 shows the bottom surface 24 of the minute projection 18. The bottom surface 24 includes the cover 8 and the paint layer 20.

FIG. 5 shows a bottom surface 24 c of a first minute projection 18 c and also shows a bottom surface 24 d of a second minute projection 18 d by an alternate long and two short dashes line. The second minute projection 18 d is adjacent to the first minute projection 18 c. In FIG. 5, an alternate long and two short dashes line 26 represents a straight line passing through the center of gravity Oc of the bottom surface 24 c of the first minute projection 18 c and the center of gravity Od of the bottom surface 24 d of the second minute projection 18 d.

In FIG. 5, an arrow P indicates a pitch. The pitch P is the distance between the first minute projection 18 c and the second minute projection 18 d adjacent to the first minute projection 18 c. The pitch P is the distance between the center of gravity Oc of the bottom surface 24 c of the first minute projection 18 c and the center of gravity Od of the bottom surface 24 d of the second minute projection 18 d. The “second minute projection adjacent to the first minute projection” is the minute projection 18 having a smallest distance to the first minute projection 18 c, among the minute projections 18 present around the first minute projection 18 c.

For each minute projection 18, one pitch P is determined. An average pitch Pav is calculated by summing the pitches P of all the minute projections 18 and dividing the sum of the pitches P by the number of the minute projections 18. The average pitch Pav is preferably not less than 10 μm and not greater than 2000 μm. With the golf ball 2 in which the average pitch Pav falls within this range, the minute projections 18 suppress rising of the golf ball 2 during flight. In this respect, the average pitch Pav is more preferably not less than 20 μm and particularly preferably not less than 30 μm. The average pitch Pav is more preferably not greater than 1500 μm and particularly preferably not greater than 1000 μm.

In FIG. 5, an arrow L indicates the distance between the first minute projection 18 c and the second minute projection 18 d adjacent to the first minute projection 18 c. For each minute projection 18, one distance L is determined. An average distance Lav is calculated by summing the distances L of all the minute projections 18 and dividing the sum of the distances L by the number of the minute projections 18. The average distance Lav is preferably not less than 5 μm and not greater than 1500 μm. With the golf ball 2 in which the average distance Lav falls within this range, the minute projections 18 suppress rising of the golf ball 2 during flight. In this respect, the average distance Lav is more preferably not less than 10 μm and particularly preferably not less than 20 μm. The average distance Lav is more preferably not greater than 1000 μm and particularly preferably not greater than 500 μm.

In FIG. 4, an arrow H indicates the height of the minute projection 18. The height H is measured along the radial direction of the golf ball 2. An average height Hav is calculated by summing the heights H of all the minute projections 18 and dividing the sum of the heights H by the number of the minute projections 18. From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the average height Hav is preferably not less than 0.5 μm, more preferably not less than 1.0 μm, and particularly preferably not less than 3.0 μm. From the standpoint that the minute projections 18 do not impair the dimple effect and therefore sufficient lift force can be obtained, the average height Hav is preferably not greater than 25 μm, more preferably not greater than 22 μm, and particularly preferably not greater than 20 μm.

In the golf ball 2, the average depth Fav of the dimples 12 and the average height Hav of the minute projections 18 satisfy the following mathematical formula (1).

Hav<Fav*0.05  (1)

In other words, the average height Hav is less than (Fav*0.05). With the golf ball 2, the minute projections 18 do not impair the dimple effect. With the golf ball 2, sufficient lift force can be obtained. In this respect, the average height Hav is more preferably not greater than (Fav*0.04) and particularly preferably not greater than (Fav*0.03). From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the average height Hav is preferably not less than (Fav*0.005), more preferably not less than (Fav*0.008), and particularly preferably not less than (Fav*0.010).

An average area Qav is calculated by summing the areas Q of the bottom surfaces 24 of all the minute projections 18 and dividing the sum of the areas Q by the number of the minute projections 18. From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the average area Qav is preferably not less than 10 μm², more preferably not less than 100 μm², and particularly preferably not less than 500 μm². From the standpoint that the minute projections 18 do not impair the dimple effect, the average area Qav is preferably not greater than 4000000 μm², more preferably not greater than 1000000 μm², and particularly preferably not greater than 300000 μm².

From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the ratio of the sum of the areas Q of the bottom surfaces 24 of all the minute projections 18 relative to the surface area of the phantom sphere 16 is preferably not less than 5%, more preferably not less than 15%, and particularly preferably not less than 20%. From the standpoint that the minute projections 18 do not impair the dimple effect, the ratio is preferably not greater than 80%, more preferably not greater than 60%, and particularly preferably not greater than 50%.

From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the total number of the minute projections 18 is preferably not less than 500, more preferably not less than 1000, and particularly preferably not less than 2000. From the standpoint that the minute projections 18 do not impair the dimple effect, the total number is preferably not greater than 500000, more preferably not greater than 300000, and particularly preferably not greater than 100000.

Preferably, the average area Sav of the dimples 12 and the average area Qav of the bottom surfaces 24 of the minute projections 18 satisfy the following mathematical formula (2).

Qav<Sav*0.016  (2)

In other words, the average area Qav of the bottom surfaces 24 of the minute projections 18 is less than (Sav*0.016). With the golf ball 2, the minute projections 18 do not impair the dimple effect. With the golf ball 2, sufficient lift force can be obtained. In this respect, the average area Qav is more preferably not greater than (Sav*0.010) and particularly preferably not greater than (Sav*0.005). From the standpoint that the minute projections 18 suppress rising of the golf ball 2 during flight, the average area Qav is preferably not less than (Sav*0.000001), more preferably not less than (Sav*0.00001), and particularly preferably not less than (Sav*0.00003).

As described above, each minute projection 18 includes the projection portion 22 of the main body 10 and the paint layer 20 (see FIG. 4). Therefore, even when the paint layer 20 is separated from the main body 10, the shapes of the minute projections 18 are substantially maintained, and the aerodynamic characteristic is substantially maintained. A special paint is not needed for forming the minute projections 18. The golf ball 2 can easily be produced.

The projection portions 22 of the main body 10 are formed simultaneously with formation of the main body 10. For the formation, a mold is used. The cavity face of the mold has a large number of minute recesses. Each recess has a shape that is substantially the inverted shape of the projection portion 22. The mold can be obtained from a master mold. One example of a method for producing the master mold is etching. During etching, a large number of minute maskings are used. By the maskings, projection portions are formed on the master mold. By the projection portions of the master mold, recesses are formed on the mold. The positions of the maskings correspond to the positions of the projection portions of the master mold, correspond to the positions of the recesses of the mold, and correspond to the positions of the minute projections 18 of the golf ball 2. The master mold can be produced by various methods other than etching. Examples of a method other than etching include laser radiation processing.

As described above, the minute projections 18 are formed on the surfaces of the dimples 12 and also on the surface of the land 14 (see FIG. 2). Therefore, the golf ball 2 has a very excellent aerodynamic characteristic.

The shape of each minute projection 18 shown in FIGS. 2 to 5 is substantially a circular column. The golf ball 2 may have minute projections 18 having another shape. Examples of the other shape include a prism, a truncated pyramid, a truncated cone, a pyramid, and a cone. The shape of each minute projection may be a part of a sphere. Each minute projection may have a shape obtained by combining a plurality of solids.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 35 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, 0.9 parts by weight of dicumyl peroxide, and 2 parts by weight of zinc octoate. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 160° C. for 20 minutes to obtain a core with a diameter of 39.7 mm. The amount of barium sulfate was adjusted such that a core having a predetermined weight was obtained.

A resin composition was obtained by kneading 50 parts by weight of an ionomer resin (trade name “Himilan 1605”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 50 parts by weight of another ionomer resin (trade name “Himilan AM7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of ultramarine blue with a twin-screw kneading extruder. The core was covered with this resin composition by injection molding to form a mid layer with a thickness of 1.0 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type solid epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 6 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. Half shells were obtained from this resin composition by compression molding. The sphere consisting of the core, the mid layer, and the reinforcing layer was covered with two of these half shells. These half shells and the sphere were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples and minute recesses on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover. Furthermore, minute projection portions having a shape that is the inverted shape of the minute recesses were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g. The golf ball has a large number of minute projections on the surface thereof. The specifications of these minute projections are shown in Table 1 below.

Examples 2 to 7 and Comparative Example 1

Golf balls of Examples 2 to 7 and Comparative Example 1 were obtained in the same manner as Example 1, except the final mold was changed and minute projections having specifications shown in Tables 2 and 3 below were formed.

Comparative Example 2

A golf ball of Comparative Example 2 was obtained in the same manner as Example 1, except the final mold was changed and a cover having no projection portion was molded.

Examples 8 to 10 and Comparative Example 3

Golf balls of Examples 8 to 10 and Comparative Example 3 were obtained in the same manner as Example 1, except the final mold was changed and dimples and minute projections having specifications shown in Table 4 below were formed.

Comparative Example 4

A golf ball of Comparative Example 4 was obtained in the same manner as Example 1, except the final mold was changed, dimples having specifications shown in Table 4 below were formed, and a cover having no projection portion was molded.

[Flight Test]

A 5-iron (trade name “SRIXON Z725”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: S, loft angle: 25.0°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 41 m/sec, and the distance from the launch point to the landing point was measured. At the test, the weather was almost windless. The average value of data obtained from 20 measurements is shown in Tables 2 to 4 below.

TABLE 1 Specifications of Dimples Curvature Dm F radius Volume Type Kind Number (mm) (μm) (mm) (mm³) I A 72 4.60 264 19.0 158.3 B 54 4.50 249 19.5 107.0 C 30 4.30 239 17.8 52.0 D 54 4.20 234 17.0 87.4 E 108 4.00 224 15.4 152.1 F 12 2.90 169 8.8 6.7 II A 72 4.60 244 22.1 146.3 B 54 4.50 229 23.1 98.4 C 30 4.30 219 21.1 47.6 D 54 4.20 214 20.1 79.9 E 108 4.00 204 18.2 138.5 F 12 2.90 149 10.6 5.9

TABLE 2 Results of Evaluation Comp. Example Example Example Example Example 1 1 2 3 4 Dimple type I I I I I Fav (μm) 238.0 238.0 238.0 238.0 238.0 Hav (μm) 15.0 10.0 5.0 5.0 5.0 Hav/Fav 0.063 0.042 0.021 0.021 0.021 Pav (μm) 280 280 280 9 2530 Sav (×10⁶ μm²) 14.17 14.17 14.17 14.17 14.17 Qav (μm²) 707 707 707 13 707 Qav/Sav 0.000050 0.000050 0.000050 0.000001 0.000050 Flight distance 205.9 207.5 209.2 207.7 207.8 (yard)

TABLE 3 Results of Evaluation Comp. Example Example Example Example 5 6 7 2 Dimple type I I I I Fav (μm) 238.0 238.0 238.0 238.0 Hav (μm) 5.0 5.0 2.0 — Hav/Fav 0.021 0.021 0.008 — Pav (μm) 780 2250 280 — Sav 14.17 14.17 14.17 — (×10⁶ μm²) Qav (μm²) 220618 3141593 707 — Qav/Sav 0.015569 0.221707 0.000050 — Flight 208.0 207.2 208.1 206.5 distance (yard)

TABLE 4 Results of Evaluation Comp. Comp. Example 3 Example 8 Example 9 Example 10 Example 4 Dimple type II II II II II Fav (μm) 218.0 218.0 218.0 218.0 218.0 Hav (μm) 12.0 10.0 5.0 1.0 — Hav/Fav 0.055 0.046 0.023 0.005 — Pav (μm) 280 280 280 280 — Sav (×10⁶ μm²) 14.17 14.17 14.17 14.17 — Qav (μm²) 707 707 707 707 — Qav/Sav 0.000050 0.000050 0.000050 0.000050 — Flight distance (yard) 205.2 206.7 208.3 207.0 205.7

As shown in Tables 2 to 4, the golf ball of each Example has excellent flight performance with a long iron. From the results of evaluation, advantages of the present invention are clear.

The aforementioned minute projections are applicable to golf balls having various structures such as a one-piece golf ball, a two-piece golf ball, a four-piece golf ball, a five-piece golf ball, a six-piece golf ball, a thread-wound golf ball, and the like in addition to a three-piece golf ball. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball having a plurality of dimples and a land, wherein the golf ball further has a large number of minute projections formed on surfaces of the dimples and/or the land, and an average depth Fav of the dimples and an average height Hav of the minute projections satisfy the following mathematical formula (1): Hav<Fav*0.05  (1).
 2. The golf ball according to claim 1, wherein an average value Pav of pitches P between the minute projections and other minute projections adjacent to the minute projections is not less than 10 μm and not greater than 2000 μm.
 3. The golf ball according to claim 1, wherein the golf ball has a plurality of rows in each of which a plurality of minute projections are aligned at equal pitches.
 4. The golf ball according to claim 1, wherein an average area Sav of the dimples and an average area Qav of bottom surfaces of the minute projections satisfy the following mathematical formula (2): Qav<Sav*0.016  (2).
 5. The golf ball according to claim 1, wherein the golf ball includes a main body and a paint layer positioned outside the main body, and the minute projections each have a shape in which a surface shape of the main body is reflected.
 6. The golf ball according to claim 1, wherein an average value Lay of distances between the minute projections and other minute projections adjacent to the minute projections is not less than 5 μm and not greater than 1500 μm.
 7. The golf ball according to claim 1, wherein the average height Hav of the minute projections is not less than 0.5 μm and not greater than 25 μm.
 8. The golf ball according to claim 1, wherein a total number of the minute projections is not less than 500 and not greater than
 500000. 